This invention relates to a process for improving the quality of heavy mineral concentrates, more particularly for the removal of and/or recovery of radioactive contaminants in such concentrates.
Bulk concentrates are usually further processed to produce individual mineral concentrates, and the presence of radioactive particles in those individual mineral concentrates may cause problems in the handling thereof. In one aspect the present invention addresses this problem by providing a process in which the bulk concentrate is subjected to flotation step to separate certain components before it is further processed to produce individual mineral concentrates. Further advantages of the process of the invention will be apparent from the following disclosure.
It will be understood that concentrations of detrital heavy minerals result from normal cycles of erosion of the land surface and economic deposits occur where the rock material has yielded sufficient quantities of the valuable mineral types and where physiography and climate have provided suitable conditions of transport and accumulation.
Deposits of heavy minerals occur widely throughout the world, with Australia, Malaysia, New Zealand, Africa, Madagascar and USA being well-known for such concentrations. The usual concentrating mechanisms are water and wind. Such deposits are now the common source of titanium minerals, primarily used for the production of the white pigment, titanium dioxide, and of zircon, a material used in ceramics and refractories.
The term "heavy mineral" has come to be associated with the higher density phases present in such deposits and is therefore used herein to refer to those minerals which have a density greater than 2.96, the density of tetrabromoethane (TBE) the liquid normally used in a sink-float operation to give preliminary estimates of valuable mineral content. A number of minerals such as tourmaline have densities between 2.96 and 3.3 and these can be quantified as "light heavy minerals" by further separation with methylene iodide at a density of 3.3.
Minerals which survive the erosive and corrosive environments commonly involved are ilmenite, rutile, zircon, monazite, xenotime, cassiterite, gold, minerals of the platinoid group, gemstones, garnet, sillimanite and tourmaline. A variety of other minerals are often associated with such deposits, e.g. leucoxene which results from the progressive oxidation and leaching of the iron present in the mineral ilmenite. Because of the progressive nature of these chemical changes, the mineralogy and chemistry of leucoxene grains vary very widely.
The common method of recovery of such minerals is by wet or dry mining, most commonly by wet dredging, followed by wet processing to recover the valuable minerals as a bulk concentrate while rejecting the bulk of minerals of no economic importance, such as quartz, as quickly as possible. The ability to achieve this objective quickly and cheaply becomes important when it is recognised that deposits containing as little as 1% valuable heavy minerals are currently treated. This wet separation usually is based on gravity methods, and use may be made of spirals, shaking tables or cone separators.
The bulk concentrate, after retreatment, if appropriate, to reduce the amount of quartz contained, is normally further processed through a relatively complicated set of unit operations to produce saleable grades of individual mineral concentrates. Commonly the first stage involves recovery of the ilmenite mineral by wet or dry magnetic separation. The concentrates generated normally require cleaning to improve the grade by rejection of other minerals entrained during the magnetic separation. Following this separation, the non-magnetic fraction must be dried, if this operation was not performed prior to magnetic separation, and then subjected to a further range of separations based on the use of electrostatic and magnetic principles. Essentially the separation of the less magnetic minerals rely upon the initial use of electrostatic separation to separate the conductors, particularly rutile and leucoxene, from the non-conducting minerals, such as zircon and monazite. The various streams resulting from the electrostatic separation then pass to units where both wet and dry separations using magnetic, gravity and/or further electrostatic separations are practised to achieve the final grades required.
A major problem encountered in such complex circuitry is the difficulty of achieving high recoveries of the minerals monazite and xenotime which are frequently present in such deposits. Both these minerals are rare earth phosphates, and apart from their economic value, they normally contain variable amounts of the radioactive elements uranium and thorium which are undesirable environmentally and in other ways. Monazite may contain up to 12% ThO.sub.2 while a typical xenotime has been reported to carry 1.85% ThO.sub.2 and 0.32% U.sub.3 O.sub.8.
Monazite and xenotime are characterised by high densities and are normally recovered, together with other heavy minerals, in the initial preconcentration circuit. However, because of the generally low levels of each and the variability of composition, subsequent separation steps involving passage through numerous items of equipment often result in incomplete recovery in final monazite or xenotime concentrates (if indeed such concentration is attempted) and the minerals disperse unevenly throughout the major concentrates, with a particular tendency to report to zircon rich fractions. However, sufficient of the radioactive particles may report to the rutile and leucoxene concentrates to cause concern to receivers responsible for down stream processing and transport and to Government authorities.
Certain of the procedures used in the production of titanium dioxide from titanium mineral concentrates, and particularly those involving the formation of the intermediate compound titanium tetrachloride, result in the further concentration of the trace amounts of radioactive elements present in such concentrates. Concern exists regarding the handling of products and equipment contaminated with radioactive materials and it is understood that U.S. Government Agencies are imposing stringent specifications on the permissible levels of radioactivity in titanium concentrates.
The environmental situation is aggravated by the preferential degradation of the rare earth phosphate minerals by attrition during wet and dry milling, as they are generally the least resistant minerals present with the potential for dust particles containing uranium and thorium to become airborne during dry separation operations. For this reason, greater emphasis is being placed on monitoring the work environments to ensure adequate levels of industrial hygiene are observed, since inhalation of radioactive dusts represents an occupational health hazard. Dust control is often necessary, requiring the installation of hooding and proper ventilation to remove the radioactive dust at the point of generation. Effective installation of equipment to achieve this is expensive and complicated by the large number of small capacity machines normally found in dry milling sections of heavy mineral separation plants.
Up to the present time, flotation has not been a favoured beneficiation procedure within the industry, however the invention herein disclosed proposes just such a procedure.
While this technique is more suitable to finer grained deposits than to the coarser beach or dune sand deposits normally treated, it can also be applied to the latter. Some limited use of flotation has been made in heavy mineral separation, including the "hot soap" flotation of zircon at Byron Bay, NSW before the introduction of electrostatic separation devices.
This invention proposes a novel approach to the problems associated with the presence in heavy mineral ore bodies of monazite and/or xenotime as accessory minerals. The economic importance of these minerals is generally minor in the context of heavy mineral production, the more important factors now being the strong need to eliminate adverse health risks associated with the presence of fine radioactive dust particles generated during milling and likely to be released into the atmosphere during dry milling and also to minimize the radioactivating level of individual mineral concentrates. The procedures described herein not only substantially eliminate such industrial hygiene risks, but can be important economically in enabling better recover of high grade concentrates of these minerals.
It was pointed out above that the complexity of the usual processing circuits and the multitude of individual items of equipment, coupled with the generally low content of the radioactive rare-earth phosphate minerals, caused an uneven distribution of such minerals throughout the final products. Consideration of such circuits has led us to the recognition of two important criteria which in current operations are not observed.
1. The operating stages should be carried out in slurry form as far as possible to minimise or eliminate dust concentration. PA1 2. Recovery of monazite and zenotime should take place as early as possible.